Man has engaged in war on the land, in the sea, in the air and in the electromagnetic spectrum. The electromagnetic spectrum has been used by the military for improved communications, guidance of aircraft and missiles and the navigation of ships and planes. A nation seeks control of the electromagnetic spectrum because of the military's increasing dependency on its use for surveillance of potential enemy forces, communications between military units, detection of enemy military forces and the guidance and control of airplanes and missiles. With a mastery of the electromagnetic spectrum, one adversary could achieve an indispensable ingredient for conquering an enemy or discouraging a potential aggressor.
One of the most important problems in electronic surveillance involves the recognition of a signal in a multisignal environment and the determination of its direction of arrival. In the past, it has been common to use a one-dimensional compressive receiver or spectrum analyzer, which decomposed the frequency of the electromagnetic spectrum into bins. If a signal appeared in a bin in which it was not previously present, a new signal would have arrived. This new signal now had to be investigated, i.e., what kind of modulation is on it, what is its direction of arrival, in order to determine if the signal was produced by a friend or a foe. Some disadvantages of the foregoing was that this procedure was a time consuming, sequential process that used: a compressive receiver for rapidly sweeping a given band of frequencies to determine the presence or existence of a signal and its frequency; a plurality of analysis receivers for analyzing the kind of modulation that is on the signal, one analysis receiver being assigned to a bin that just received a new signal and some antenna receivers that acted as an interferometer by determining the direction of arrival of the incoming signal by detecting the signal's phase shifts.
The analysis receivers that were used in the aforementioned system required a large amount of space and were expensive. Hence, there was a limited number of analysis receivers, i.e., six (6) that were utilized in prior art systems. Thus, if the new signal birth rate was too high, there would not be enough analysis receivers to process all of the incoming signals.